Visualization of the Phase Propagation within Carbon-Free Li4Ti5O12

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Visualization of the Phase Propagation Within Carbon-Free LiTiO Battery Electrodes 4

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Chunjoong Kim, Young-Sang Yu, Benjamin Moyon, Chutchamon Sirisopanaporn, Thomas J. Richardson, and Jordi Cabana J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11459 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016

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The Journal of Physical Chemistry

Visualization of the Phase Propagation within Carbon-Free Li4Ti5O12 Battery Electrodes Chunjoong Kim,†,‡,+ Young-Sang Yu,†,‡,§ Benjamin Moyon,‡ Chutchamon Sirisopanaporn,‡ Thomas J. Richardson,‡ and Jordi Cabana*,†,‡

†Department

of Chemistry, University of Illinois at Chicago, Chicago, IL 60607, USA

‡Environmental

Energy Technologies Division, Lawrence Berkeley National Laboratory,

Berkeley, CA 94720, USA

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Phone: 312-355-4309

Present Address +

C.K.: Department of Materials Science and Engineering, Chungnam National University,

Daejeon, 34134, Korea §Y.-S.

Yu.: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA

94720, USA

ACS Paragon Plus Environment

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ABSTRACT

Electrochemical reactions in a battery involve transport of ions and electrons between the electrode and the electrolyte and current collector, respectively. In Li-ion battery electrodes, this dual functionality is attained with porous composite electrode structures that contain electronically conductive additives. Recently, the ability to extensively cycle composite electrodes of Li4Ti5O12 without any conductive additives generated questions of how these structures operate, which could be used to design architectures with other materials that reduce the amount of additives that do not directly store energy. Here, the changes occurring in carbonfree Li4Ti5O12 electrodes during lithiation was studied with a combination of ex-situ and operando optical microscopy and microbeam X-ray absorption (µ-XAS). The measurements provide visualization of how lithiated domains percolate through the thick (~40 µm) structure after only 1% depth of discharge, followed by a second wave propagation starting with regions in closest contact with the current collector, progressing toward regions in contact with the bulk electrode. These results emphasize the interplay between the electronic and ionic conductivity of the phases involved in a battery reaction and their formation in localized areas in the electrode architecture. It provides new insight that could be used to refine the design of these architectures in order to minimize transport limitations while maximizing energy density.


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1. INTRODUCTION Li-ion batteries are the most important energy storage devices today because of their high energy/power density, negligible memory effect, low self-discharge, and long life.1-3 They are currently under close scrutiny as storage systems for a renewable energy-based electricity grid and have an incipient presence in the market of electric vehicles, boosted by growing concerns over the finite supply and environmental consequences of fossil fuels. Still, breakthroughs must be achieved to implement Li-ion batteries in these applications at the required scales.4-6 As a result, intensive research efforts have been devoted to find better materials and design architectures to improve the performance of those currently being used.7-10 Battery electrodes are currently built in the form of a 50-100 µm thick and porous composite of the redox active material, a conductive additive, and a polymeric binder in order to ensure the dual transport of electrons and ions required to complete an electrochemical reaction.11 Component assembly during the formation of these composites is poorly controlled, resulting in architectures that, while cheap, quick and efficient to build, handicap the inherent performance of the active material12 and require post-fabrication procedures of quality control that add overhead in time and cost.13 Achieving full control of the fabrication of electrodes requires understanding how materials function at the electrode level, where kinetic conditions always prevail, in comparison with ideal, even close-to-equilibrium conditions of cycling. The energy density of a battery can be raised by increasing the mass loading of active material per unit area in the electrode, for instance, by reducing the ratio of support components.14 However, when poorly electronically conducting materials are used as electrodes, performance losses are predicted if the corresponding additive is removed, particularly if the electrode material is a wide band gap semiconductor. Examples are Ti4+ oxides, which are highly


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electronically insulating because they have empty 3d orbitals. Phases like the different TiO2 polytypes, Li2Ti3O7, and especially, Li4Ti5O12 are alternatives to graphitic carbonaceous materials that comparatively provide higher rate capability and safety when used as negative electrodes.15-17 Li4Ti5O12 is considered a good choice for the negative electrode in applications where power and durability are critical. It has classically been considered to react with Li+ through a two-phase reaction to produce Li7Ti5O12 at ~1.55 V vs. Li+/Li0, for a theoretical specific capacity of 175 mAh g-1, because of the prominent plateau of potential observed during the electrochemical reaction.18,19 However, recent work suggests that segregation of domains of different lithium concentrations occurs at such a short scale (below 10 nm)20 that complete mixing into pure solid solutions is possible in certain conditions.21,22 In any case, the negligible volume change (∆V = ~0.2%) induced between the two end phases brings about excellent cycling stability and rate performance.18,19 So far, the poor conductive properties of Li4Ti5O12 in the pristine state have led researchers to focus on synthesizing or processing it in composites with carbon to ensure good performance.23,24 However, recent work has demonstrated that Li4Ti5O12,25-27 as well as other titanium oxides,28,29 can be made to cycle very stably without conducting agents in the composite electrode. The key to this behavior is the rapid propagation of lithiated domains throughout the composite structure. These domains both contain mixed Ti3+/Ti4+ (d1/d0) states, which create electronic carriers,27 and show enhanced Li diffusion with respect to the initial state.30 Percolation of these domains throughout the electrode architecture at very low overall levels of lithiation creates a conductive network in-situ, inducing a sharp increase in electronic conductivity that enables the reaction to complete lithiation.27 The existence of percolation was inferred from the detection by Raman spectroscopy of electronically conducting states at the top


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layers of the electrode, away from the current collector, as well as a sharp drop in its electrical impedance. Recently, conductive scanning probe measurements refined this insight by noting that the presence of electronically conducting states at the surface of a thin (>1 µm) film was not laterally homogeneous, but formed percolation “channels”.31 However, the propagation of lithiated domains in a real, thick composite electrode has not yet been visualized. Analytical tools exist to detect compositional or structural changes at various length scales, using X-ray, laser, or visible light.32-35 Optical microscopy is ideally suited for this system, as well as other titanium oxides,36 because the color of the active phase changes from white to dark blue upon lithiation.26,27 However, it only offers qualitative information, and, thus, the relative ratios of the species cannot be quantified. In turn, X-ray absorption spectroscopy (XAS) can directly provide electronic signatures, by which oxidation state and local coordination can be obtained without the need for the domains to be crystalline. Thus, raster scans of focused hard Xray microbeams combined with energy tunability across the K-edge absorption of transition metals (i.e., microbeam X-ray absorption, µ-XAS) constitute an ideal means to quantitatively visualize, at high resolution, chemical compositional maps for electrodes at intermediate states of charge.37-39 In this study, we visualized the electrochemical lithiation of Li4Ti5O12 in composite electrodes free of conductive additives, primarily in cell configurations where the electron and ion supply (from the current collector and bulk electrolyte, respectively) were located on opposing sides. The phase front was traced by ex-situ and operando optical microscopy measurements by leveraging the large color contrast between end phases, while composition could be quantitatively identified by studying µ-XAS across the titanium K-edge. As a result, the propagation of lithiated domains proposed by us in an earlier report was confirmed,26 and lateral


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compositional domains were identified as being due to inhomogeneities in the distribution of active material, binder and pores in the composite electrode.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. Li4Ti5O12 was synthesized via solid state reaction using Li2CO3 (Sigma Aldrich, #62470) and anatase TiO2 (Sigma Aldrich, #248576) as starting materials, mixed in stoichiometric ratios in a mortar, followed by annealing at 850 ºC for 5 h under ambient atmosphere. The cooling rate was set to be 2 ºC min-1. 2.2 Characterization. Powder X-ray diffraction (XRD) was performed on a D2 Phaser (Bruker AXS) diffractometer equipped with Cu Kα (λavg = 1.5418 Å). Scanning electron microscopy (SEM) images were obtained with a JSM-7500F field-emission microscope (JEOL) in gentle beam mode at 1 kV and 20 mA. High resolution digitalized optical microscopy was carried out on a MXG-2500REZ microscope (Hirox Co., Ltd.) with the magnification from 35 to 2500ⅹ. Xray absorption spectroscopy was carried out at the Ti K-edge at beamline 2-3 in the Stanford Synchrotron Radiation Lightsource (SSRL) at the SLAC National Accelerator Laboratory (California, USA). The beamline is equipped with Kirkpatrick-Baez (K-B) optics that were used to focus the beam to 2 × 2 µm2. In order to obtain pixel-by-pixel fluorescence yield, the focused beam is raster scaned onto 500 × 500 µm2 field-of-view with 45o incident angle. All single pixel fluorescence yields and X-ray absorption spectra (XAS) were simultaneously collected with a Vortex fluorescence detector and ionization chamber while tuning the incident X-ray energy across the Ti K-edge.40 Four fluorescence yield maps were collected at characteristic fluorescence features (at 4971, 4976 and 4987), which have clear contrast corresponding to the


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oxidation state of Ti, and the post-edge region (5017 eV) for the normalization. The intensity of the incident X-ray was normalized using the flux measured in the first ionization chamber positioned between the sample and source. Since, the fluorescence yield at the post-edge region isn’t affected by the oxidation states of Ti, other fluorescence maps were normalized by the map at 5017 eV to regularize the fluorescence yields with respect to variations in sample (e.g. sample thickness). To obtain the chemical phase distribution of the 50% DOD sample, least-squares linear combination (LC) fitting with normalized standard spectra of Li4Ti5O12 and Li7Ti5O12 were performed for each single-pixel fluorescence yield. To enhanced the visibility, the color scale of the chemical map was limited in the range of 9.4 to 11.4 % of DOD. To confirm the accuracy of the chemical map, the full spectra were also measured at the most oxidized and reduced single pixels (Figure S1 in the Supporting Information file), which resulted in compositions equivalent to ~9.4% (= Li~4.28Ti5O12) and ~11.4% DOD (= Li~4.34Ti5O12), respectively. The quality of LC fit for the full spectra (Figure S1 in the Supporting Information file) can be confirmed with the R-factor defined as, R = ∑ ( data-fit )

2

∑ ( data )

2

. All data

processing, including normalization of fluorescence spectra and LC fitting, was performed using SIXPACK and SMAK MicroToolkit.41,42 All cell assembly and sample manipulation was performed in an Ar-filled glovebox and the ex-situ samples for µ-XAS measurements were sealed in Mylar 400 (Dupont, 50 µm thick polyester films) to protect them from deleterious contact with the ambient atmosphere. 2.3 Electrode Preparation and Evaluation of Electrochemical Properties. The working electrodes were prepared by mixing Li4Ti5O12 and polyvinylidene difluoride (PVDF) in Nmethyl pyrrolidone (NMP) with a mass ratio of 90 : 10. When homogeneity was reached in the slurry, it was cast onto aluminum foil, followed by drying under vacuum at 110°C overnight.


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The dried electrodes had an areal loading of ~ 8.4 g cm-2, and were calendared with a line pressure of 600 kgf cm-1 with a roll press (IRM Lab Mill Table Model). The calendared electrodes were ~ 45 µm thick, which corresponds to ~40% porosity. Electrodes were punched into disks with a diameter of 3/8" and transferred to an Ar-filled glove box (both water and oxygen ≤ 0.1 ppm). 2032 type coin cells were assembled with Li4Ti5O12, high-purity lithium foil (Sigma Aldrich, #320080), and Celgard 2500 as working electrodes, counter electrodes, and separators, respectively. The electrolyte was composed of 1 M LiPF6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 50:50 (Novolyte Technologies). Charge-discharge measurements of Li4Ti5O12 vs. Li+/Li0 were recorded on a Biologic VMP3 potentiostat at room temperature at C/10 between 0.9 V to 2.5 V, where C = 175 mA g-1 corresponded to the current needed to reach theoretical capacity of the electrode in an hour. Ex-situ samples for optical microscopy studies were carefully harvested from cells at the desired depth of discharge (DOD), which is the percentage of battery capacity that has been discharged expressed as a percentage of maximum specific capacity, states in the Ar-filled glove box and placed in the air tight sealed sample stage. Samples with 0, 1, 10, 25, 50, 75, 100% DOD were prepared, which corresponded to the discharge capacity of 0, 1.75, 17.5, 43.75, 87.5, 131.25, and 175 mAh g-1. For cross-sectional view, samples were sliced with blades and aligned vertically. The operando cells contained a piece of Li4Ti5O12 electrodes (4.6 × 3.6 mm2) and Li foils as the working and the counter/reference electrode, respectively; their preparation was carried out in the glove box. After tightly sealing the sample stage, the electrolyte was injected with a syringe into the cell. The cell was discharged to 0.9 V at C/10 with image recording every 30 s. All potentials in this report are referenced to the Li+/Li0 couple.


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2.4 Sample Thickness Calculation. Absorptivity can be affected by volumetric density (ρ), thickness (t), and mass absorption coefficient (µ) of the materials, as expressed by Beer’s law.43 I = I0 exp(-µρt)

(Eq. 1)

where, I0 and I are the intensities of incident and transmitted X-rays, respectively. At the postedge region (at 5017 eV), the absorption coefficient of the 50% DOD sample was estimated based on the relationship of µ = ∑ wi µ i , where wi and µi is the weight fraction and mass i

absorption coefficient of the ith constituent. Under the estimation of uniform density throughout the electrode, the attenuation of the transmitted beam intensity (I) was highly correlated with the thickness of the electrode. To extract thickness information from the transmitted intensity, the following parameters were considered, after calibration by taking into account the transmittance of Mylar films that were used seal the electrodes44: µelec = 368.48 cm2/g, estimated from µi of Li, Ti, and O at 5017 eV,45 assuming an average homogeneous composition Li5.5Ti4O12. ρelec = 2.09 g/cm3, assuming 40 % of porosity. 2.5 Operando Optical Microscopy. A movie for real time information of phase propagation kinetics was acquired by operando optical microscopy, Movie S1 in the Supporting Information file, using a lithium metal half cell containing a piece of Li4Ti5O12 electrode, sealed inside the air-tight setup.


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3. RESULTS Li4Ti5O12 was synthesized by a simple solid state reaction using Li2CO3 and TiO2. Characterization of the material properties, including its very low electronic conductivity, has been reported in previous work.26 Phase purity and crystal structure were confirmed by X-ray diffraction (XRD) as shown in Figure 1a; the pattern matched a Fd 3 m spinel structure (JCPDS card number #49-0207) without any notable crystalline impurities. Calendared 45 µm thick carbon-free composite electrodes with 40% porosity were prepared to attain a specific capacity close to the theoretical value and good durability.26 Despite the thickness of the electrode and the absence of conducting agents, stable and extended galvanostatic cycling at C/10 rate (C = 175 mA g-1, theoretical capacity) could be achieved (Figure 1b), as described in earlier reports.25, 26 Li4Ti5O12 electrodes showed a clear first-order transition via two phase reaction between Li4Ti5O12 and Li7Ti5O12 at 1.55 V, resulting in the specific capacity of ~170 mAh g-1. The electrodes showed very little capacity loss after 30 cycles with ~100% coulombic efficiency. Surface and cross-sectional images of the as-prepared electrodes by scanning electron microscopy (SEM) revealed some lateral topological and morphological irregularities, which were due to the large size (up to several micrometers) and irregular shape of the oxide particles (Figures 1c and 1d).


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Figure 1. a) XRD pattern of Li4Ti5O12. b) Voltage-capacity curve of carbon-free Li4Ti5O12 electrodes, cycled in Li metal half-cells at C/10 rate. SEM images of the composite electrodes c) in-plane and d) along their cross section.

The absence of carbon in the electrodes made it possible to follow the transition between white Li4Ti5O12 and dark blue Li7Ti5O12 (ref. 26) using a visible light digital microscopy, combined with an airtight sample stage (Figure 2a) that enabled both ex-situ, with samples at different depths of discharge (DOD) and operando measurements (see experimental section). Figures 2b and 2c show images taken perpendicular to the top (i.e. opposite to the current collector, Figure 1d) and the cross-section, respectively, of electrodes harvested from coin cells


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ranging from pristine (Li4Ti5O12) to 100% DOD (Li7Ti5O12). In this cells, the bulk electrolyte (ion supply) and connections to the external circuit (electron supply) are located on opposing sides of the electrode (Figure 1d). In both cases, growth of dark domains was observed due to the formation of Ti3+ as lithiation proceeded. The top surface image at 1% DOD showed that the whole electrode changed from white to pale blue in the early stages of lithiation. At higher reduction levels (> 10% DOD samples in Figures 2b and 2c), dark blue domains formed fast, and lateral color inhomogeneity developed. The size of dark domains increased upon lithiation, with the 100% DOD sample being almost completely dark blue, except for a few light speckles dispersed along the top surface. Further details on the transformation were revealed upon close inspection of the cross-sectional view of a sample harvested at 25% DOD (Figure 2d). This electrode cleanly peeled off from the current collector, revealing inhomogeneities across the thickness of the electrode. The region of the electrode originally facing the current collector (bottom fraction) was found to be mostly dark, thus indicating extensive lithiation, whereas the rest of the electrode, toward the top surface in contact with the bulk of the electrolyte in the cell, was still pale blue. As illustrated in the scheme, these observations are indicative of the phase front propagating from the bottom (current collector side) to the top surface (bulk electrolyte side) of the electrode.


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Figure 2. a) Scheme of the airtight sample stage for optical microscopy. b) and c) shows top surface and cross-sectional view images, respectively, of the ex-situ samples at different DODs. Red arrows in c) indicate the thickness direction of the electrode. d) Zoomed-in cross-sectional image for 25% DOD, which shows dark domains propagated from the current collector side (bottom surface). The positions of current collectors are indicated as blue arrows in c) and a hatch in d).

Complementary quantitative studies were performed by XAS at the Ti K edge. The bulk spectra of the two end members, pristine (Li4Ti5O12) and 100% DOD (Li7Ti5O12), shown in


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Figure 3a, were used as standards to determine composition in mixed states. There is a 2.2 eV rising edge shift, from 4982.7 eV to 4980.5 eV, upon reduction of Ti4+ to Ti3+, as measured from the first root in the first derivatives of the near-edge region. The spectra consist of three characteristic regions, pre-edge peaks at ~4971 eV, shoulder peaks at ~4976 eV, and white-line peaks at ~4987 eV, which are noted as A, B, and C in Figure 3a, respectively. Weak pre-edge peaks, A, are attributed to the electric quadrupole (1s 3d transition) via 3d-4p hybridization; their position and intensity are strongly related to the distortion in centrosymmetry of the TiO6 octahedra, which affects 3d-4p orbital mixing.46, 47 The shoulder peaks, B, are assigned to the 1s 4p transition involving a shakedown process caused by ligand-to-metal charge transfer (LMCT), while the white line peaks, C, correspond to purely dipole-allowed 1s 4p transition without a shakedown.48 The intensity of feature A decreased with lithiation of Li4Ti5O12 due to the decrease in structural disorder, or, in other words, an increase in the centrosymmetric nature of the TiO6 octahedra. This decreased distortion in the local structure by lithiation is consistent with previous structural studies, where Li7Ti5O12, the lithiated phase, was found to show an ordered rock salt structure.49 Concurrently, the features in region B are more pronounced in Li7Ti5O12, which indicates that the LMCT process is more prominent. The intensity in region C also increased with lithiation, indicating a decrease in 3d–4p orbital mixing due to the formation of more centrosymmetric TiO6 octahedra, which increased the probability of the dipole transition (1s 4p).


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Figure 3. a) Ti K-edge XAS spectra of Li4Ti5O12 and Li7Ti5O12, produced electrochemically. b) Oxidation map of 50% DOD produced from µ-XAS image collected in fluorescence mode. Images were taken at the energy points indicated as A-D in a). The first three images (at A, B, and C) were selected to estimate the oxidation state of samples while the last image (at D) was selected for normalization purposes. The green and red color indicate the most (11.4% DOD, Li~4.34Ti5O12) and less (~9.4% DOD, Li~4.28Ti5O12) oxidized phases, respectively.

By virtue of the beam focusing capabilities (down to 2 × 2 µm2), coupled with a high precision in sample positioning (± 50 nm), Ti oxidation state maps were acquired. Data was obtained at four relevant energy states, denoted by arrows in Figure 3a. It included a point in the post-edge region, D, around 5107 eV, for background normalization purposes. Since the fluorescence yield (FY) is highly localized within a probe depth of a few µm based on the absorption coefficient and density of the composite electrodes and measurement geometry,50 the chemical information was primarily sensitive within the top fraction of the electrode. The oxidation maps were collected perpendicular to the surface of the 50% DOD electrode that was


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in contact with the electrolyte, and the pixel-by-pixel fluorescence intensity changes with X-ray energy were fitted with a linear combination (LC) of the two standard spectra using a leastsquare method (Figure 3b). The Ti oxidation map revealed a uniform, yet low level of lithiation (~ Li4.31Ti5O12) occurring near the top surface of the electrode at 50 % DOD. Full spectra were also measured at the most oxidized and reduced single pixels (Figure S1 in the Supporting Information file), which resulted in compositions equivalent to ~9.4% (= Li~4.28Ti5O12) and ~11.4% DOD (= Li~4.34Ti5O12), respectively. The accuracies of the LC fits are represented by the Rfactors, which are less than 1 × 10-3 for the both cases (see experimental section). This low extent of lithiation near the top surface of the sample could be attributed to lateral and/or vertical inhomogenities of the reaction. The field-of view (FOV, 500 × 500 µm2) was likely large enough to cover both white and dark blue domains in the optical image (50% DOD in Figure 2b). Thus, the discrepancy between average composition of the electrode and the value measured using µXAS is primarily ascribed to vertical inhomogeneity due to the bottom-up mechanism of phase propagation modeled in Figure 2c. The discrepancy between the lateral inhomogeneity observed in the optical microscopy (Figure 2b) and the homogeneity of oxidation states in the µ-XAS map (Figure 3b) of the top fraction of the electrode, exposed to the electrolyte, at 50% DOD could be attributed to the different optical skin depth between techniques, which can be generated by the morphology and topology of the architecture. In other words, dark Ti3+-rich domains close to the current collector could be visible in an optical image taken from the surface facing the bulk electrolyte, as in Figure 2b, if covered by lightly colored Ti3+-poor domains, due to their transparency. This situation appears to apply to 50% DOD. This complex phase distribution vis-à-vis the topology of the electrode can be evaluated from the response of the sample to the X-ray beam. Indeed, the


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intensity of the beam transmitted at 5017 eV through the 50% DOD sample during XAS measurement noticeably fluctuated, as shown in Figure 4a, which corresponds to the same area in Figure 3b. Because the image was collected well above the absorption edge, this attenuation is correlated to the absorptivity of the sample, regardless of chemical state. The average thickness derived from the transmitted beam intensity was estimated to be ~40 µm, which matches well with the actual thickness based on the SEM results of Figure 1d. In order to account for the compositional heterogeneity, a model was built where the composition of the bottom half, i.e. ~20 µm, was Li7Ti5O12, the rest being a composition proximate to the pristine state. The corrected mass coefficients (µLi4Ti5O12 = 376.62 cm2/g and µLi7Ti5O12 = 360.35 cm2/g) were again applied for each pixel, see Equation 1 in the Experimental Section. The resulting electrode topology is shown in Figure 4b. The electrode revealed a lateral microstructural inhomogenity with root-mean-squared roughness (Rrms) of ~4.41 µm. Indeed, thinner portions of the electrode corresponded to slightly higher values of lithiation.


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Figure 4. a) Transmitted beam intensity in a 500 × 500 µm2 area of the 50% DOD electrode. b) 3D visualization of the electrode surface structure, revealing significant roughness, Rrms = ~4.41 µm.

Real time information concerning the kinetics of the phase propagation was acquired by operando optical microscopy in Figure 5, using a half cell containing a Li4Ti5O12 electrode and a lithium foil counter electrode, sealed inside the airtight sample stage in Figure 2a. A movie obtained from the operando cell, showing the color changes, can be found as Movie S1 in the Supporting Information. Optical images at relevant DOD states were extracted from the complete image set, as presented in Figure 5a. The resulting voltage-composition trace is shown in Figure 5b. The electrochemical signals from the operando cell were close to that obtained in standard coin cells (Figure 1b), with the electrode achieving full capacity (3 mol Li in Li4Ti5O12) at the 0.9 V discharge cutoff. In order to obtain detailed insight, the degree of darkness was quantified for the region circle-marked in the 0% DOD image, and plotted with the voltage curve in Figure 5b. The area was carefully chosen to be representative of the phase propagation in the


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whole electrode, where inhomogeneity seemed to be minimized. The color change demonstrated strong correlation with electrochemical response. The largest color change took place in the very early stages of lithiation (up to ~10% DOD, see dashed line in Figure 5b), indicating the formation of mixed states of Ti3+/Ti4+ up to the surface, which result in a sharp increase in electronic conductivity. Electronic percolation throughout the electrode was thus rapidly achieved as the voltage dropped from OCV (~2.77 V) to 1.5 V. Upon completion of electronic percolation throughout the electrode, phase propagation from the current collector side to the electrolyte side occurred. During the phase propagation, the color of the electrode got only slightly darker, with subtle changes observed after intercalation of 1.5 Li+ (50% DOD). Finally, the darkest color was achieved at 0.9 V, yet, as in the case of the ex situ experiments, some lighter speckles were still observed. Bubbling was observed during the reduction of Li4Ti5O12. It is possible that this phenomenon is related to the macroscopic observations of gassing during operation of these electrodes in large format cells,51-54 the origin of which is still under debate.


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Figure 5. a) Representative optical images taken operando during discharge of Li4Ti5O12 in a Li metal half cell sealed in the holder shown in Figure 2a. b) Voltage-capacity curve (red line) obtained in the operando experiment, and degree of darkness (blue symbols), which quantifies the color change in the electrode.

4. DISCUSSION The inhomogeneities in the degree of lithiation observed both ex-situ and operando in electrodes that were discharged slowly (C/10) are at odds with a mechanism where only a solid solution between end members is formed28 because such solid solution transformation involves homogeneity in chemical states throughout the electrode states, especially in equilibrated, ex-situ samples. Overall, the results confirm that lithiated, Ti3+-rich phases propagate through the thick composite electrode in the very early stages of reaction (~1% DOD), which was very rapidly followed by the uneven advance of darker phases toward the whole electrode structure. Thus, they are in agreement with the phase progression proposed based on Raman and impedance spectroscopy.26 The first wave of lithiation in carbon-free Li4Ti5O12 involved the formation of


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percolating domains with high electronic conductivity, thus enabling the complete lithiation of the electrode. This initial process can be viewed as introducing an electronically conductive additive in situ, which significant levels of percolation achieved with only 1% of electronicaaly conducting phase. Recently, the formation of percolation channels within individual grains was identified to play a role in the early propagation of lithiated domains in thin (

Visualization of the Phase Propagation within Carbon-Free Li4Ti5O12

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